The evolution and technical development of adhesive compositions to adapt to ever changing substrates, bonding conditions and working environments is an ongoing and challenging process for researchers in the adhesives field. Due to the recently heightened awareness and concern for the environment, a common thread among all research projects involving adhesive formulations is the goal of developing a water-based system that will avoid the use of volatile solvents which may be harmful to the environment. Many of these research projects are targeted towards developing adhesive compositions which are useful for bonding elastomeric materials.
There are many types of elastomeric materials and each elastomeric material may have unique properties that must be dealt with in order to develop an adhesive which will effectively bond that elastomeric material. For example, many natural rubber compositions are cured by utilizing various sulfur compounds. These sulfur-cured elastomeric materials have common properties due to the nature of the sulfur curing process. Various elastomeric materials such as silicone rubbers, nitrile butadiene rubbers, and fluoroelastomers can also be cured by utilizing polyol or peroxide compounds. Polyol-cured elastomers and peroxide-cured elastomers are typically utilized in the production of various seals and gaskets as high performance specialty elastomers which are capable of withstanding extreme conditions such as high temperature and/or organic fluid environments. Adhesive compositions that effectively bond sulfur-cured elastomeric materials do not necessarily exhibit the same level of performance when utilized to bond non-sulfur-cured elastomeric materials such as the high performance specialty elastomers described above. In general, non-sulfur-cured elastomeric materials are relatively difficult to bond and represent a substantial challenge to researchers in the adhesives industry.
One example of an adhesive previously developed for the bonding of peroxide-cured silicone rubbers is described in U.S. Pat. No. 3,022,196. This adhesive composition utilizes a mixture of vinyl triethoxysilane and gamma-aminopropyl triethoxysilane in a proportion of about five parts by weight of the vinyl silane per part by weight of the aminopropyl silane. The vinyl silane is not water-soluble and therefore it is necessary to use alcohols or other organic solvents to prepare the adhesive formulation. Various acidic compounds are recommended to adjust the pH of the formulations (and apparently to catalyze hydrolytic reaction of the silanes' alkoxy groups with water which may be present in a minor amount).
A need accordingly exists for an environmentally safe, aqueous adhesive composition which can effectively bond various materials including non-sulfur-cured elastomeric materials, particularly high-performance specialty elastomeric materials such as polyol- or peroxide-cured elastomers.